It's why the playoffs are the playoffs and the regular season is the regular season. I only caught the first quarter of the Mavs against the Nuggets but it was evident that the Nuggets were the aggressor, taking it right at the Mavs, and reciprocating it with hard physical defense, especially on Dirk Nowitzki.

In the post-game news conference, Mavs head coach Rick Carlisle acknowledged as much that the Mavs needed to step up their aggressiveness, so that Dirk Nowitzki will get more than just four free-throws:

We’re going to have to raise our level of aggression if that’s what’s going to get us to the free throw line... I’ll look at that closely, and if the referees were right they were right. But he’s being played very physically, away from the floor where the rules are different than in post play. We’ll look at it and if there’s a complaint to be made we’ll talk to the league about it.

Here's the video clip of the news conference,

The Nuggets have notched a first round knockdown in Game 1. How will the Mavs respond now in Game 2? The biggest question in this series is whether Dirk will step up and play through the physicality. This is one of those career-defining moments for Dirk. Back in 2006, after the Heat took it to the Mavs in the NBA Finals, Dirk famously shrunk away. As Dwyane Wade got more aggressive, Dirk withered away. There's reason to believe that the Mavs are ready to make that leap. If this was 2006, Mark Cuban would have popped off into his usual ranting and whining about the refs, so far nothing. Carlisle had and presumably has learned from his failures with the Pistons and the Pacers.

As for the Nuggets, I was very impressed with how they played. Especially how they responded when Carmelo got into foul trouble, and how they did not rely on Billups having a huge game. This is a tough, physical team, that plays the style of basketball that can potentially go all the way. The biggest question mark for the Nuggets is team chemistry. There are a lot of personalities there. They will face adversity at some point in this playoffs, how will they respond? Will they blow up, or will they come together.